At present, the channelization codes in a CDMA system are usually allocated at random or in turn during channel allocation. For example, in a multi-sector Time Slot CDMA system employing smart antennas, the channelization codes are allocated at random or according to the sequence number of the channelization codes during channel allocation. The CDMA code resources in a multi-sector Time Slot CDMA system with smart antennas are shown in FIG. 1, which is a schematic diagram of the code channel resources in a Time Slot CDMA system. It has N allocable time slots, and each time slot has Q allocable channelization codes. The channel allocation process of the system comprises the following steps:
1. Measuring the interference power or related physical quantities of each time slot in real time.
2. Allocating a user to an optimum one of the 1 to N time slots, for example, the time slot with the minimum interference power.
3. Which channelization code of the optimum time slot occupied by the user is used at random or in turn, for example, in the order of serial number of the channelization codes from small to large: 1, 2, . . . , Q.
After the channelization code is determined, the spread spectrum code of the channel is obtained by calculating the dot product of the cellization code (usually it is scrambling code) allocated during system planning and the occupied channelization code. The spread spectrum code is used for distinguishing between different channels in the same cell, and it is sent to a user along with the signaling or data. Scrambling code can change the orthogonal property of a channelization code. Since there are only 16 channelization codes in a multi-sector Time Slot CDMA system with smart antennas, strong correlativity may exist between the different spread spectrum codes of adjacent sectors, or even identical spread spectrum codes may appear. Thus, strong same-frequency interference may be introduced.
For a multi-sector system, the radio channel from a user to an adjacent sector is almost the same. As a result, the correlativity of spread spectrum codes will increase the system interference greatly. Moreover, for a system employing short spread spectrum codes, coincident spread spectrum codes may appear. In such a case, the sector edge may cause the demodulation of two users having a coincident code to fail.